Development of thermal barrier coating systems from Al microparticles. Part I: Influence of processing conditions on the mechanisms of formation

Abstract

This work presents the mechanisms of formation on pure nickel of full thermal barrier coating systems (aluminide coating, thermally grown oxide and thermal barrier top coating) from micro-sized Al particles dispersed in a slurry annealed in different atmospheres (Ar, synthetic air, water vapour and mixtures thereof). The simultaneous formation of nickel aluminides and of a thermal barrier made of sintered hollow alumina spheres involved self-propagating high-temperature synthesis in all conditions. However, the microstructures and adherence of the top coatings changed markedly with either a diffusion step (700°C-2 h) or a complete heat treatment (700°C-2 h + 1100°C-2 h) depending on the atmosphere. Whereas fast consumption of Al occurred in Ar to form the nickel aluminides, synthetic air and water vapour fostered the peripheral oxidation of Al micro-sized particles that impeded the release of Al and its diffusion towards the substrate. This resulted in heterogeneous diffusion layers but thicker top coatings with better sintering and thicker alumina shells.

Development of Thermal Barrier Coating Systems from Al Microparticles—Part II: Characterisation of Mechanical and Thermal Transport Properties

“…As the indenter rapidly reached the aluminide surface, it was not possible to properly measure the contribution of the top foam coating itself. Such poor mechanical strength was also observed in previous works and is at the origin of the study that sought to improve the strength of the top foam [11].…”

“…Annealing in a quasi-inert Ar atmosphere leads to the fast consumption of the Al from the slurry HS and DS microparticles to form the aluminide coating and the subsequent collapse of the remaining emptied shells from the top coating. The collapse was reported to occur from the shrinkage of the thin alumina shells when γ-Al 2 O 3 transforms into α-Al 2 O 3 after exposure to temperatures above 1000 • C [7,11,18,19]. The greater collapse of the foam top coating for the DS-Ar than for the HS-Ar sample is probably due to the thinner passive shell of the DS microparticles (Figure 1a,f and Figure 2a,f).…”

“…The mechanisms of formation of the coatings characterised here were fully discussed in our previous work [11]. Therefore, the microstructural aspects of the coatings obtained in the different annealing conditions are only briefly recalled here.…”

“…Previous studies that investigated the role of the microparticles' properties [7], the mechanisms of coating formation [8] or the influence of the annealing gases [9,10] on the formation of such coatings showed that it was possible to obtain different foam top coatings by varying the parameters of the annealing conditions or the microparticles of the slurry itself. In an attempt to thicken the shell and enhance the sintering of the hollow spheres, hence to improve the mechanical strength of the insulative foam, previous research aimed at controlling the annealing atmosphere to promote the growth of the oxide shells of the original microparticles while enabling enough Al to react with the Ni substrate to form the diffusion coating [11]. It was demonstrated that annealing in air and in water vapour thickened the oxide shells but hampered the diffusion of Al into the Ni substrate.…”

The Interdiffusion Behavior of NiCoCrAlYHf Coating Deposited by Arc Ion Plating on Carburized Ni-Based Single Crystal Superalloy

“…Meanwhile, the thickness of the HY5 coating mildly reduced. For a 1000 h oxidation time, the TGO spalled and became discontinuous [ 21 ].…”